A control device of a continuously variable transmission for a vehicle (hereinafter, continuously variable transmission) is well known that has a pair of variable pulleys with variable effective diameters of an input-side variable pulley (a primary pulley, a primary sheave) and an output-side variable pulley (a secondary pulley, a secondary sheave) and a transmission belt wound around between the pair of the variable pulleys and that respectively controls an input-side thrust force (a primary thrust force) in the primary pulley and an output-side thrust force (a secondary thrust force) in the secondary pulley to set an actual gear ratio to a target gear ratio while preventing a slip of the transmission belt. This corresponds to a speed change control device for a belt type continuously variable transmission described in Patent Document 1. For such a continuously variable transmission, for example, a target thrust force on the secondary side (a target secondary thrust force) is set to a limit thrust force to a slip (a required secondary thrust force) which is a minimum required thrust force to prevent a belt slip from occurring in the secondary pulley. A target thrust force on the primary side (a target primary thrust force) is set to the sum (total thrust force) of a balance thrust force (steady thrust force) balanced with the target secondary thrust force based on a thrust force ratio (=secondary thrust force/primary thrust force) for maintaining the target gear ratio and a shifting thrust force (transient thrust force) for achieving a target shift speed when a gear ratio is changing. Oil pressures to each of the pulleys are controlled so as to acquire respective set thrust forces, thereby achieving the target gear ratio while preventing a belt slip from occurring.
For example, it is assumed that a downshift of the continuously variable transmission is performed. Since the shifting thrust force on the primary side is a negative value in the case of downshift, the target primary thrust force to be set easily falls below a limit thrust force to a slip (hereafter referred as a slip limitation thrust force) (a required primary thrust force) which is a minimum required thrust force to prevent a belt slip from occurring in the primary pulley and may cause a belt slip. Since the shifting thrust force on the primary side is a positive value in the case of upshift, the target primary thrust force to be set hardly falls below the required primary thrust force; however, the target primary thrust force may fall below the required primary thrust force and cause a belt slip depending on the balance thrust force on the primary side etc. With regard to such a target primary thrust force falling below the required primary thrust force, Patent Document 1 proposes that when the target primary thrust force is smaller than the required primary thrust force, the slip limitation thrust force is set as the target primary thrust force on the primary side while the total thrust force of the balance thrust force for shifting and the shifting thrust force is set as the target secondary thrust force on the secondary side, thereby achieving an intended shift while preventing a belt slip from occurring by using a minimum required pulley thrust force. Therefore, in Patent Document 1, it is determined whether the target primary thrust force for shift control corresponding to the target secondary thrust force set to the slip limitation thrust force (required secondary thrust force) is smaller than the slip limitation thrust force (required primary thrust force) and a function of assuring the belt slip prevention and a function of achieving an intended shift are interchanged between the primary side and the secondary side based on the determination result, thereby achieving the shift and the belt slip prevention with a minimum thrust force.